ERR/E1.5: catch sema (pure-failable slice) + error-set match subjects

`expr catch [e] BODY` consumes a failable's error inline. Pure-failable slice
(value-carrying `-> (T, !)` catch deferred to E2's tuple ABI).

- lowerExpr `.catch_expr` -> lowerCatch; inferExprType `.catch_expr` ->
  operand's success type (void for pure-failable).
- lowerCatch: operand must be failable (else "catch requires a failable
  expression"); pure-failable LHS only (value-carrying bails to E2). Eval
  operand -> err tag; condBr to handle (error) / merge (success). In handle:
  child scope binds `e` to the tag (typed as the error set), lower body
  (block or expr); if the body didn't diverge, br merge. Result is void.
  `catch` needs no failable enclosing function — it handles the error locally.
- All four body forms work: block, no-binding `catch { }`, bare-expr, and
  the match-body `catch e == { case ... }`. Re-raise (`raise e`) and diverging
  bodies (`return`) rely on E1.3 / E1.4c.

Also: lowerMatch now supports error-set subjects — `case .X` resolves to the
global tag id (was the arm index, dispatching wrong), and the switch operand
is the error-set value (its u32 tag) directly rather than via enumTag. This
is what the catch match-body form (and a plain `if e == { case .X }`) needs.

Tests: examples/226-catch.sx (block / no-binding / match-body / re-raise /
diverging body / success-skip; exit 18), examples/227-catch-rejections.sx
(operand-not-failable; exit 1). Gates: zig build, zig build test,
265/265 examples.

examples/226-catch.sx

@@ -0,0 +1,64 @@
+// `catch` on a pure-failable LHS (ERR step E1.5). `expr catch [e] BODY`
+// consumes the error inline: on failure it binds the tag to `e` (optional)
+// and runs BODY; on success the result is void (a `-> !` LHS has no success
+// value). BODY may diverge (`return` / `raise` — typed `noreturn`, E1.4c) or
+// fall through. `catch` needs no failable *enclosing* function — it handles
+// the error locally. All four body forms appear below: block, no-binding
+// block, match-body (`== { case ... }`), and the selective handle + re-raise
+// pattern. Value-carrying `-> (T, !)` catch (binding the success value) lands
+// with the tuple ABI in E2.
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+must :: (n: s32) -> !E {
+    if n < 0  { raise error.Bad; }
+    if n == 0 { raise error.Empty; }
+    return;
+}
+
+// Diverging body — returns from `classify` on error.
+classify :: (n: s32) -> s32 {
+    must(n) catch e {
+        if e == error.Bad   { return 1; }
+        if e == error.Empty { return 2; }
+        return 9;
+    };
+    return 0;                               // must(n) succeeded
+}
+
+// Match-body form — sugar for `catch e { if e == { case ... } }`.
+mclassify :: (n: s32) -> s32 {
+    must(n) catch e == {
+        case .Bad:   return 11;
+        case .Empty: return 22;
+        else:        return 99;
+    };
+    return 0;
+}
+
+// Selective handle + re-raise (failable enclosing fn; `raise e` is the
+// variable form). Swallows Bad → success; re-raises everything else.
+handle_some :: (n: s32) -> !E {
+    must(n) catch e {
+        if e == error.Bad { return; }       // swallow → success
+        raise e;                            // re-raise the rest
+    };
+    return;
+}
+
+main :: () -> s32 {
+    r : s32 = 0;
+    must(-1) catch e { if e == error.Bad { r = r + 1; } };  // Bad → +1
+    must(5)  catch { r = r + 100; };                         // success → body skipped
+    r = r + classify(0);                                     // Empty → 2
+    r = r + classify(8);                                     // success → 0
+    he := handle_some(0);                                    // Empty re-raised
+    if he == error.Empty { r = r + 4; }                      // +4
+    hb := handle_some(-1);                                   // Bad swallowed → success
+    if hb == error.Bad { r = r + 50; }                       // not taken
+    r = r + mclassify(-1);                                   // Bad → 11
+    print("catch result: {}\n", r);                          // 1+2+4+11 = 18
+    return r;
+}

examples/227-catch-rejections.sx

@@ -0,0 +1,13 @@
+// `catch` rejection (ERR step E1.5): the operand must be failable. Unlike
+// `try` / `raise`, `catch` needs no failable enclosing function — it consumes
+// the error locally — so the only stable rejection is a non-failable operand.
+// The positive cases live in `examples/226-catch.sx`.
+
+#import "modules/std.sx";
+
+plain :: () -> s32 { return 0; }
+
+main :: () -> s32 {
+    plain() catch e { return 1; };   // error: operand has type s32 (not failable)
+    return 0;
+}